Photosensitive printing plate having a light-sensitive coating consisting of a colloid, a phosphate, and a chromium compound



Patented Jan. 6, 1953 UNITED STATES PATENT OFFICE Frederick H. Frost,Portland, and Frederic E. Brinnick, Westbrook, Maine, assignors to S. D.Warren Company, Boston, Mass., a corporation of Massachusetts NoDrawing. Application March 1, 1948,

f Serial No. 12,475

7 Claims.

This invention relates to the art of photolithography and moreparticularly to a sensitizing composition and to a photosensitive platecomprising a surface coating of said composition.

The invention resides primarily in the incorporation of certainwater-soluble phosphate salts in sensitizing compositions consistingessentially of a protein and a chromate or dichromate as will be morefully described hereinafter.

The usual preparation of plates for photolithog-raphy may be describedas follows. A suitable base or support such as a grained metal plate ora coated paper surface is coated or sensitized with a light-sensitivefilm of an aqueous solution containing a protein such as egg albumin anda sensitizing chemical such as ammonium dichromate. In place of eggalbumin other proteins such as animal glue, casein and blood albumin maybe used and the ammonium dichromate may be replaced by other solubledichromates or chromates such as those of sodium and potassium. Thesensitized plate, after being dried, is placed under a mask or negativehaving a transparent pattern in an opaque field, and it is exposedthrough the mask to strong light such as an arc-light. The action of thelight on the photo-sensitive film, while not wholly understood, is saidto tan the film, and the film becomes water-insoluble in the exposed ortanned areas. In the unexposed areas under the opaque portions of themask or negative the film i not afiected by the light and remainswater-soluble. The exposed plate is next wiped over with greasydeveloping ink which coats the entire surface. Finally the plate iswashed with gentle rubbing in water which loosens the water-solubleareas of film and removes them together wtih the ink overlying thoseareas, but leaves on the plate the exposed, water-insoluble areastogether with the ink overlying them. After the preceding treatment theplate is ready to use for printing. The unexposed areas are kept wetwith aqueous solution and an inking roller coated with lithographicprinting ink is rolled across the entire surface. Ink is taken up by thegreasy image but is repelled by the water on the non-imaged areas. Whenthe inked plate is brought into contact with another surface ittransfers the ink thereto in a pattern reverse to that on the plate. Thesurface so printed upon may be a paper sheet, but in most cases is anofiset blanket which in turn transfers the print to a paper sheet tocomplete the printing operation in a pattern identical with that of theoriginal plate. The plate is then repeatedly dampened, inked,

and printed from until the desired number of copies have been made.

There are several objectionable features in the present-day art. In thefirst place the treatment with developing ink and the subsequent washingoperation together constitute troublesome and time-consuming procedure.Even when metal plates are used great care must be exercised to ensureboth that all the unexposed film is removed from the plate and that thedeveloping ink does not soil the bare non-printing areas of the plate.On the other hand, if the application of developing ink is omitted andthe unexposed areas are washed off directly the image is liable to bevery slow in taking sufficient ink for good printing, and in some casesthe image may actually be obliterated in the washing process while theunexposed areas are being washed ofi. Moreover, if the film is notwashed completely oil the unexposed areas the plate is liable to scum orbecome dirty while it is being used, so that unsatisfactory prints willbe obtained therefrom. Complete removal of unexposed film and overlyingink is difficult enough even when metal plates are used; it is doublydifficult in the case of paper-base plates which have a more or lessporous surface.

The practice of the present invention avoids some of the difiicultiespreviously met with in the art of sensitizing plates forphotolithography. For instance, the invention makes entirely feasiblethe omission of the customary development of the image by theapplication of developing ink prior to removal of the film in unexposedareas although the customary development with ink may be carried out asusual if desired. Whether or not developing ink is used on the plateprior to removal of the unexposed film, the invention greatlyfacilitates removal of the unexposed film and if developing ink ispresent also aids in its removal. Moreover, in addition to expeditingthe removal of unexposed film from the surface one feature of theinvention mitigates or substantially nullifies the ill effects of anyunexposed film inadvertently left onthe surface of the plate, as will beexplained hereinafter.

According to the invention an aqueous mixture or solution is preparedcontaining a protein such as egg albumin, casein, animal glue or thelike, a soluble chromate or dichromate such as ammonium dichromate. anda soluble phosphate, such as monoammonium ortho-phosphate, having a pHvalue higher than the isoelectric point of the protein, and theresulting mixture is applied in an even coating to the surface of asuitable base such as a metal, coated paper, or equivalent printingplate base, to sensitize the said surface to actinic' radiation; Thesensitized plate so produced may be used in conventional manner fromthis point forward, but it possesses several advantages over similarplates conventionally sensitized with albumin and dichromate withoutaddition of phosphate.

The sensitized plate may be exposed, under a mask, to a source ofactinic rays and may then, if desired, be coated with developing ink.When a so-treated plate is washed with wate the unexposed film andoverlying developing ink are very easily removed. It is not necessary toimmerse the plate beneath running water as in common practice, but it issufficient merely to wipe the surface with a sponge wet with water.

The improvement in the facility with which the unexposed areas can becleaned when phosphate is present is very, marked even when metal platesare used. In the case of paper-base planographic plates, from which theunexposed sensitized film is ordinarily removed with considerabledifliculty, the effect of phosphate in the film is even more remarkable.Especially noteworthy are the results obtained when one uses coatedpaper-base planographic printing plates which contain a water-solubledivalent metal salt such as zinc acetate in the coating layer, such asare disclosed in the applications of Stephen V. Worthen, Serial No.747,138, filed May 9, 1947, and Serial No. 758,215, filed June 30, 1947,both now abandoned. When such plates are sensitized with the usualalbumin-dichromate sensitizer containing no phosphate no amount ofscrubbing with water will remove from the unexposed areas the yellowdiscoloration caused by the action of the dichromate. On the other hand,when phosphate is present in the sensitizing layer in sufficientquantity a, single passage of a wet sponge over the unexposed surfacesuifices to expose the color of the original coated base and gentleswabbing cleans the unexposed areas completely. Part of the explanationfor this result probably is the pore pluggingaction of insolublephosphate formed by interaction of the phosphate with the divalent metalsalt as disclosed in an application of Frederick H. Frost, Serial No.791,447, filed December 12, 1947, now Patent No. 2,559,610, but almostcertainly it is also partly due to the detergent action of thephosphate.

The ease with which unexposed areas are cleaned in the practice of theinvention makes severescrubbing unnecessary and accordingly the exposedand imaged areas likewise escape the effects of severe scrubbing.Consequently, it is found feasible to omit entirely the application ofdeveloping ink to the plate before washing. In the past it has indeedbeen possible to omit the developing step before washing, but the degreeof scrubbing previously required to clean the unexposed areas was sosevere that imaged areas likewise were liable to be weakened or damaged.Accordingly, in the past it has generally been considered unsafe to omitthe use of developing ink prior to washing. Now, however, use ofphosphate according to the invention makes it possible to eliminateentirely the troublesome and disagreeable application of developing inkprior to washing without at the same time endangering the quality of theimage.

The defect commonly designated as albumin scum is well-known in thephotolithographic art. This defect is caused by failure to remove allthe albumin or other protein from the unexposed area of the plate. Whenthe plate containing traces of albumin in the unexposed areas is run ona press the albumin gradually picks up ink and causes tone in thesupposedly blank areas. This difficulty is common whether metal platesor paper-base plates are used. Use of phosphate according to theinvention greatl decreases trouble from albumin scum, since removal ofall the albumin is greatly facilitated by the action of the phosphate.

Another advantage derived from the incorporation of phosphate in theprotein-dichromate sensitizing solution resides in the fact thathydrophilic gums or polysacharrides or similar hydrocolloids can beincorporated in the solution. Hitherto it has been the general beliefthat admixture of a gum such as gum arabic with a protein sensitizingsolution was fatal to successful use Of the solution inphotolithography, common experience having indicated that such asensitizing mixture of albumin and gum would not, after exposure, retainan image sat1sfactorily. Surprisingly it has now been found that asensitizing coating comprising solubilized protein, soluble chromate ordichromate, soluble phosphate and water-soluble non-protein hydrocolloidin quantity up to equality by weight with the protein present can beused with very satisfactory results on plates designed for medium runsof up to 2,000 copies. Inclusion of the soluble nonprotein hydrocolloidin the film is a considerable safeguard against development of albuminscum, for the non-protein hydrocolloid not only makes the unexposed filmeasier to wash away, but also in case traces of unexposed film areinadvertently left behind it makes such traces definitely morewater-receptive and less ink-receptive and hence very definitely acts tokeep the blank portions of the plate clean during the printingoperation.

While, as has been stated, a sensitized coating yielding an image withlife satisfactory for a medium run plate can be made by the use of asensitizin solution containing in addition to the soluble phosphate andchromate or dichromate,

equal parts by weight of protein and soluble non-' protein hydrocolloid,it should be realized that inclusion of such a high proportion ofsoluble non-protein hydrocolloid definitely does act to cut down ordecrease the potential image life of the plate. Consequently, it isgenerally desirable to use a smaller proportion of the non-proteinhydrocolloid in the sensitized coating. It is found that quantities ofsoluble non-protein relatively less viscous hydrocolloid ranging fromabout 5% to about 25 of the weight of protein present are very efiectivein facilitating wash-off and preventing albumin scum as beforedescribed, and at the same time such quantities are much lessdetrimental to image life.

The preferred range of 5% to 25% of soluble non-protein hydrocolloidbased on the protein content of the film applies to the less viscoushydrocolloids such as gum arabic and mesquite gum among the natural gumsor polyvinyl alcohol and sodium carboxy methyl cellulose among thesynthetic hydrocolloids. The more highly viscous use with chromates anddichromates in usual photolithography, such as albumin, (egg albumin,blood albumin, etc.) animal glue, casein, soy protein and other animaland vegetable proteins. In practice, egg albumin is the preferredprotein as it is in ordinary photolithographic procedure.

The sensitizing chemical generally preferred for use under the inventionis, as also in usual commercial photolithography, ammonium dichromatebecause of the greater speed with which it or compositions containing itreact under light and the shorter exposure which is consequentlyrequired for formation of an image on the plate. Other solublechromates, e. g. sodium and potassium chromates and dichromates andammonium chromates are usable, however, provided longer exposure timecan be afforded. Ratios of dichromate to albumin under the invention arein general the same as are customary in ordinary photolithography.

The optimum ratio of chromate or dichromate, calculated as ammoniumdichromate, to protein, calculated as egg albumin, has been found to beabout 1 to 3 by weight when the sensitizer is applied to a metal orother non-absorbent and non-reactive base. However, when a coatedpaper-base is used it may be found advantageous to use a sensitizingsolution having a dichromatealbumin ratio of 2 to 3, and generally it ispermissible to use a ratio of 1 to 1. Paper-base plates will tolerate ahigher dichromate-albumin ratio than will metal plates, withoutdeveloping trouble from excess crystallization of the salt. Whenphosphate is present in the sensitizing solution it is not necessary touse as high a dichromate-albumin ratio as is otherwise required on apaper-base plate, but apparently no disadvantage results from using ahigh ratio if desired. The excess dichromate used apparently is merelywashed off in later treatment of the plate when phosphate is used, forin general there is no permanent discoloration of the paper-base plateby the dichromate as is seen when. no phosphate is used.

The soluble phosphate included in the sensitizing solution according tothe invention should when dissolved have a pH value above theisoelectric point of the particular protein used (3.8 for egg albuminand 4.5 for casein); otherwise the unexposed film will be diiicult toremove from the plate. On the other hand, since films having a pH valueof not over 7 are generally desired in the interest of shortening therequired exposure time, the preferred phosphates to use are those whichin cooperation with the chromium sensitizer used give a solution havinga pH value of not over 7.0. For absolute certainty in this respect it isadvantageous to use acidic phosphates which by themselves give solutionshaving pH values of not over 7.0. It is not, however, essential to usean acidic phosphate. for the chromium compound chosen for use in thesolution may be one such as ammonium dichromate which exerts a bufferingaction to keep the pH value of the solution below 7.0. In such a case aphosphate as alkaline as trisodium orthophosphate may be used with goodresults, though naturally it is possible to use so much of a highlyalkaline phosphate that the resulting solution may become too alkalinefor satisfactory use. In the proper proportions an alkaline phosphatemay be included in a solution of ammonium dichromate and albumin toyield a coating which when remoistened will be slightly acidic and verysatisfactory for the purpose of the invention. In general watersolubleinorganic phosphates and water-soluble organic phosphates are suitablefor use.

While the particular base to which the sensitizing solution of theinvention is to be applied is not a part of the invention, it may bementioned in passing that the coated paper plates referred to above andcontaining solubl salts of divalent metals, especially zinc acetate, areexceptionally suitable for use with the phosphatecontaining sensitizingsolution of the present invention.

The invention is considered to reside in: (l) a coating composition forsensitizing photo-sensitive plates comprising an aqueous mixture of aprotein, a soluble chromate or dichromate, and a soluble phosphatehaving in dilute solution a pH value higher than the isoelectric pointof the protein, and if desired a non-protein hydrocolloid in amount notexceeding the weight of protein present, and (2) a photo-sensitive platecomprising a suitable base and a sensitized coating consisting of thedried residue of the composition described under (1).

The invention is illustrated by the following specific examples, allparts being by weight:

1. A sensitizing solution was made containing 30 parts of egg albumin,10 parts of ammonium dichromate and 7 parts of mono-ammoniumortho-phosphate in 420 parts of water with enough ammonia added to givethe solution a pH value of 8.

A thin film of this solution was applied by means of a plate whirler toa grained zinc lithographic plate and dried. The so-sensitized plate wasexposed under a negative in conventional manner. The exposed plate wasthen wiped with a cotton pledget wet with water, which cleaned theunexposed areas very easily. The cleaned plate was then put on an offsetpress where it ran satisfactorily.

2. A sensitizing solution was made containing 30 parts of egg albumin, 5parts of mesquite gum, 12 parts of ammonium dichromate, and 5 parts ofmono-ammonium ortho-phosphate, in 500 parts of water with sufficientammonia to give the solution a pH of about 8. This solution was spreadby means of a whirler on a grained aluminum lithographic plate, anddried. The sensitized plate was exposed under a negative in conventionalfashion. It was then coated with developing ink and then wiped cleanwith a wet cotton sponge which cleaned the unexposed areas readily. Theplate was then run satisfactorily on an offset press.

3. A sensitizing solution containing 36 parts of egg albumin, 6 parts ofmesquite gum, 18 parts of ammonium dichromate and 15 parts of diammoniumortho-phcsphate in 660 parts of water with enough ammonia to give a pHvalue of 8.7 was sponged over a commercial paper-base planographic platehaving a hydrophilic coating of the type disclosed in the Worthenapplications referred to above comprising casein, inert filler, guargum, and zinc acetate. The late was then dried and exposedconventionally through a negative. The exposed plate was wiped over witha wet cotton sponge which cleaned the unexposed areas readily. It wasthen run satisfactorily on an offset press.

4. A sensitizing solution containin 30 parts of egg albumin, 5 parts ofgum arabic, 19 parts of ammonium dichromate, and 12 parts ofmonoammonium ortho-phosphate in 640 parts of water with enough ammoniato give a pH of 9 was applied by a sponge to the surface of a paper-'base planographic plate having a surface coating of polyvinyl alcohol,and dried. The plate was then exposed conventionally under a negative.The exposed plate was wiped over with a cotton swab wet with water whichcleaned the unexposed areas readily. The plate was then runsatisfactorily on an offset press.

5. A sensitizing solution containing egg albumin 30 parts, low viscositysodium carboxymethyl cellulose 4 parts, ammonium dichromate 18 parts,disodium ortho-phosphate 12 parts, in water 630 parts with enoughammonia to give the solution a pH value of 8 was applied to a paper-baseplanographic plate having a surface coating of sodium alginateinsolubilized by subsequent treatment with zinc acetate as disclosed insaid Worthen applications. The plate was then dried and exposed under anegative to an arc-light. The exposed plate was sponged with a cottonswab wet with water which readily cleaned the unexposed areas. The platewas then run satisfactorily on an oifset press.

6. A sensitizing solution containing 30 parts of casein dissolved byammonia, mesquite gum '7 parts, ammonium dichromate 15 parts, anddiammonium ortho-phosphate 12 parts, in 630 parts of water withsufficient ammonia to give a pH value of 8.5 was applied by a sponge toa paper-base planographic plate having a surface coating of sodiumalginate insolubilized by a subsequent treatment with zinc acetate asdisclosed in said Worthen applications. This was dried and then exposedthrough a negative in conventional manner. The exposed plate was spongedwith a wet cotton swab which cleaned the unexposed areas readily. Thecleaned plate was run satisfactorily on an offset press.

'7. A sensitizing solution containing low viscosity animal glue 30parts, mesquite gum 6 parts, ammonium dichromate 16 parts, andmonoammonium ortho-phosphate 10 parts, in 650 parts of water, withammonia to give a pH value of 8.5 was applied by a swab to a paper-baseplanographic printing plate having a surface coating comprising zinccarboxymethyl cellulose and zinc acetate as disclosed in said Worthenapplications. The plate was dried and exposed through a negative to anarc-light. The exposed plate was then sponged with a swab wet with waterwhich readily cleaned the unexposed areas. The plate then ransatisfactorily on an offset press.

8. A sensitizing solution containing egg albumin 30 parts, low-viscositysodum cal-boxy methyl cellulose 3 parts, ammonium dichromate 16 parts,and mono-ammonium ortho-phosphate 10 parts, in 540 parts of water withsufficient ammonia to give the solution a pH value of about 9 wasapplied by a swab to a paper-base planographic printing plate having abase-coat of casein and clay and a wash-coat of guar gum and zincacetate as disclosed in said Worthen applications. The plate was driedand exposed under a negative in conventional manner. The exposed platewas sponged with water which cleaned the unexposed areas readily, andthe plate then ran satisfactorily on an ofiset press.

9. A sensitizing solution containing egg albumin 30 parts, ammoniumdichromate 16 parts, dextrin '7 parts, and mono-ammonium orthophosphate10 parts in water 530 parts with ammonia to give pH value of 8.5 wasapplied by a sponge to a plate like that used in Example 8, and dried.The sensitized plate was exposed through 8 a negative as usual. It wasthen sponged with water which cleaned the unexposed areassatisfactorily, and it was then run on an offset press with goodresults.

10. A sensitizing solution containing egg albumin 30 parts, ammoniumdichromate 16 parts, and mono-sodium ortho-phosphate 11 parts, in water540 parts, with ammonia to give pH value of 7.8 was applied to the samebase as used in Example 8, and dried. The sensitized plate was exposedthrough a negative in the usual manner. The exposed plate was wiped witha sponge wet with water, which cleaned the unexposed areas easily. Theplate was then run on an offset press satisfactorily.

In the foregoing examples, we have given the pH values of the coatingsolutions. It will be understood, however, that these are not the pHvalues of the resulting dried coatings because the pH values of thesolutions are influenced by the presence of ammonia which generally isadded to act as a preservative and which vaporizes when the coatings aredried. The pH value of the coating should, as stated, be within therange from the isoelectric point of the protein used in the coating, e.g. 3.6 in the case of egg albumin and 4.5 in the case of casein up toabout pH 7. The pH value of the coating is determined by wetting thecoating with water and determining the pH of the resulting solution andcorresponds to the pH value of the dilute solution of the salt mixtureused in the coating. Coatings above 9.0 in pH value are too alkaline tobe used.

The quantity of phosphate in the coating composition, e. g. its ratio tothe protein or to the chromate or dichromate ingredients of thecomposition may vary within a considerable range. As appears from thespecific examples the ratio by weight of the phosphate to the proteinvaries from about 15% to about 50%. The quantity of phosphate to be usedwill depend, however, upon the particular phosphate used, the characterof the protein, the quantity of non-protein hydrocolloid, if any, in thecomposition, the dilution of the composition, the character of thesupporting surface, etc. and may be varied independently of such factorsto vary the ease with which the unexposed portion of the film is removedfrom the supporting surface. The most useful range of phosphateconcentrations, reckoned as mono-ammonium ortho-phosphate on the weightof the sensitizing solution is from 0.5% to 3.0%, with the lowerconcentrations more applicable to metal plates and the higherconcentrations to paperbase plates.

Various other sensitizing solutions have been made up essentially likethat of Example 10 except for the omission of ammonia and replacement ofthe 11 parts of mono-sodium ortho-phosphate by each of the followingphosphates:

Example 11 .Sodium meta phosphate, 11 parts.

Example 12.Sodium acid pyrophosphate, 11 parts.

Example 13.-Trisodium ortho-phosphate, 11 parts.

Example 14.-Pentaethyl potassium tripolyphosphate, 11 parts.

Example 15.-Ammonium hexametaphosphate, 11 parts.

Example 16.-Mono-isopropyl sodium orthophosphate, 11 parts.

Example 17.Ethyl ammonium ortho-phosphate, 11 parts.

Example 18.Monoethanolamine acid orthophosphate, 11 parts.

Example 19.Dimethylamine acid ortho-phosphate, 11 parts.

Each of the foregoing solutions of Examples 11-19 was applied to thesame kind of coated paper plate used in Example 8 and further treated inthe same way. In every case the unexposed areas cleaned up readily witha wet swab and remained clean when the plate was run on a press. Theimaged areas tool: ink readily on the press and gave a satisfactorynumber of good prints.

An efiect of the phosphate content of the sensitizer is that it rendersthe exposed portion of the coating more water-shedding. This effeet isnot marked when the film-forming ingredient of the coating issubstantially all protein because such coatings possess this quality toconsiderable extent but when the coating contains a considerableproportion of a non-protein hydrocolloid which has little or none of thewater-shedding quality the effect of the phosphate in rendering theexposed coating more water-shedding is more noticeable.

We claim:

1. A photo-sensitive plate comprising a base having a planographicprinting surface and a coating thereon consisting of the dried residueof an aqueous mixture consisting essentially of a light sensitivecolloid comprising a protein, a water-soluble chromium compound of thegroup consisting of chromates and dichromates and a water-solublephosphate a dilute aqueous solution of which has a pH value above theisoelectric point of the protein, the ratio of chromium compound toprotein being within the range from 1 part of chromium compound to from1 to 3 parts of protein and the ratio of phosphate to protein beingwithin the range of from to 50 parts of phosphate to 100 parts ofprotein.

2. A photo-sensitive plate as defined in claim 1 in which the phosphateis an acid salt of orthophosphoric acid of the group consisting of themonoand di-alkali metal and ammonium orthophosphates.

3. A photo-sensitive plate as defined in claim 1 in which the phosphateis mono-ammonium ortho-phosphate.

4. A photo-sensitive plate as defined in claim 1 in which the phosphateis a salt of pyro phosphoric acid.

5. A photo-sensitive plate as defined in claim 1 in which the lightsensitive colloid comprises a water-soluble non-protein hydrocolloid inquantity amounting to from about 1.5 to by weight of the protein contentof the composition.

6. A photo-sensitive plate as defined in claim 1 in which the protein isegg albumin, the chromium compound is ammonium dichromate, the phosphateis mono-ammonium ortho-phosphate in quantity amounting to from about 20%to about 50% by weight of the protein content of the composition and thecomposition comprises a Water-soluble non-protein hydrocolloid inquantity amounting to from about 5% to 25% of the weight of the proteincontent of the composition.

7. A photosensitive plate comprising a paper base having a planographicprinting surface, a first coating thereon consisting essentially of apigment, a hydrophilic adhesive and a watersoluble divalent metal salt,and a second coating superposed upon said first coating, said secondcoating consisting essentially of a protein, a water-soluble chromiumcompound of the group consisting of chromates and dichromates and awater-soluble phosphate a dilute aqueous solution of which has a pHvalue above the isoelectric point of the protein, the phosphateamounting to from 15% to 50% of the protein and the chromium compoundamounting to from one part thereof to from 1 to 3 parts of the protein.

FREDERICK H. FROST. FREDERIC E. BRINNICK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 55,592 Willis June 12, 18661,618,931 Kammerer Feb. 22, 1927 2,544,877 Bryce Mar. 13, 1951 FOREIGNPATENTS Number Country Date 228,377 Great Britain Feb. 5, 1925 408,097Germany Jan. 2, 1925 598,799 Germany June 18, 1934

1. A PHOTO-SENSITIVE PLATE COMPRISING A BASE HAVING A PLANOGRAPHICPRINTING SURFACE AND A COATING THEREON CONSISTING OF THE DRIED RESIDUEOF AN AQUEOUS MIXTURE CONSISTING ESSENTIALLY OF A LIGHT SENSITIVECOLLOID COMPRISING A PROTEIN, A WATER-SOLUBLE CHROMIUM COMPOUND OF THEGROUP CONSISTING OF CHROMATES AND DICHROMATES AND A WATER-SOLUBLEPHOSPHATE A DILUTE AQUEOUS SOLUTION OF WHICH HAS A PH VALUE ABOVE THEISOELECTRIC POINT OF THE PROTEIN, THE RATIO OF CHROMIUM COMPOUND TOPROTEIN BEING WITHIN THE RANGE FROM 1 PART OF CHROMIUM COMPOUND TO FROM1 TO 3 PARTS OF PROTEIN AND THE RATIO OF PHOSPHATE TO PROTEIN BEINGWITHIN THE RANGE OF FROM 15 TO 50 PARTS OF PHOSPHATE TO 100 PARTS OFPROTEIN.